Mitel 3300 Switch Port Configuration

[Linkawc32]

Hello,
I have a rather difficult customer who will refuse to change anything on his network unless there is a good reason. While working on his equipment, I discovered the switchport that the 3300 is plugged into is both tagged and untagged on the voice vlan. I tried explaining to him that the 3300's switch port should be untagged on voice only. Does anyone have a detailed answer to why its bad to have tagged and untagged traffic on the same vlan hitting the 3300?

Thanks in advance! I'm pretty new to VoIP/Mitel so please forgive me!

[Dogbreath]

Before you go back to him, you want to be absolutely certain that it is the way you think it is - I'm yet to come across a switch that allows the same VLAN to be both tagged and untagged on the same port. What would the switch be doing when configured like that? Sending two copies of every frame, one tagged and one untagged? Doesn't make any sense, to me.

[VinceWhirlwind]

I have HP MSR routers that allow you to tag and untag the voice VLAN on the same port, but I can't think of any Cisco, Procurve or Juniper switch I have ever worked on that would allow that. I think 3COM switches do some weird stuff like this.
 
It means the controller will be seeing two copies of every packet. I doubt the controller will read the tagged ones if its own interface is set to untagged, but if it did, you could get some weird stuff happening.
I'd call it a security issue, personally, but those hardware controllers do some strange things when they reboot or during an upgrade so maybe he's done what he's done for a reason.